Improve the error message when asserting to a type that is not

comparable to the original.

Also improve the error message for implicit conversion of a symbol to a
string in a template literal, which previously shared the error message
with type assertions.

Fixes microsoft#25539.  Addresses microsoft#25870.

Author: mattmccutchen

src/compiler/checker.ts

@@ -20041,7 +20041,8 @@ namespace ts {
             if (produceDiagnostics && targetType !== errorType) {
                 const widenedType = getWidenedType(exprType);
                 if (!isTypeComparableTo(targetType, widenedType)) {
-                    checkTypeComparableTo(exprType, targetType, errNode, Diagnostics.Type_0_cannot_be_converted_to_type_1);
+                    checkTypeComparableTo(exprType, targetType, errNode,
+                        Diagnostics.Conversion_of_type_0_to_type_1_may_be_a_mistake_because_neither_type_sufficiently_overlaps_with_the_other_If_this_was_intentional_convert_the_expression_to_unknown_first);
                 }
             }
             return targetType;
@@ -21515,7 +21516,7 @@ namespace ts {
             // in tagged templates.
             forEach(node.templateSpans, templateSpan => {
                 if (maybeTypeOfKind(checkExpression(templateSpan.expression), TypeFlags.ESSymbolLike)) {
-                    error(templateSpan.expression, Diagnostics.Type_0_cannot_be_converted_to_type_1, typeToString(esSymbolType), typeToString(stringType));
+                    error(templateSpan.expression, Diagnostics.Implicit_conversion_of_a_symbol_to_a_string_will_fail_at_runtime_Consider_wrapping_this_expression_in_String);
                 }
             });
 

src/compiler/diagnosticMessages.json

@@ -1196,7 +1196,7 @@
         "category": "Error",
         "code": 2351
     },
-    "Type '{0}' cannot be converted to type '{1}'.": {
+    "Conversion of type '{0}' to type '{1}' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.": {
         "category": "Error",
         "code": 2352
     },
@@ -2421,6 +2421,10 @@
         "category": "Error",
         "code": 2730
     },
+    "Implicit conversion of a 'symbol' to a 'string' will fail at runtime. Consider wrapping this expression in 'String(...)'.": {
+        "category": "Error",
+        "code": 2731
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

tests/baselines/reference/arrayCast.errors.txt

@@ -1,4 +1,4 @@
-tests/cases/compiler/arrayCast.ts(3,23): error TS2352: Type '{ foo: string; }[]' cannot be converted to type '{ id: number; }[]'.
+tests/cases/compiler/arrayCast.ts(3,23): error TS2352: Conversion of type '{ foo: string; }[]' to type '{ id: number; }[]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
   Type '{ foo: string; }' is not comparable to type '{ id: number; }'.
     Object literal may only specify known properties, and 'foo' does not exist in type '{ id: number; }'.
 
@@ -8,7 +8,7 @@ tests/cases/compiler/arrayCast.ts(3,23): error TS2352: Type '{ foo: string; }[]'
     // has type { foo: string }[], which is not assignable to { id: number }[].
     <{ id: number; }[]>[{ foo: "s" }];
                           ~~~~~~~~
-!!! error TS2352: Type '{ foo: string; }[]' cannot be converted to type '{ id: number; }[]'.
+!!! error TS2352: Conversion of type '{ foo: string; }[]' to type '{ id: number; }[]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 !!! error TS2352:   Type '{ foo: string; }' is not comparable to type '{ id: number; }'.
 !!! error TS2352:     Object literal may only specify known properties, and 'foo' does not exist in type '{ id: number; }'.
 

tests/baselines/reference/asOperator2.errors.txt

@@ -1,8 +1,8 @@
-tests/cases/conformance/expressions/asOperator/asOperator2.ts(1,9): error TS2352: Type 'number' cannot be converted to type 'string'.
+tests/cases/conformance/expressions/asOperator/asOperator2.ts(1,9): error TS2352: Conversion of type 'number' to type 'string' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 
 
 ==== tests/cases/conformance/expressions/asOperator/asOperator2.ts (1 errors) ====
     var x = 23 as string;
             ~~~~~~~~~~~~
-!!! error TS2352: Type 'number' cannot be converted to type 'string'.
+!!! error TS2352: Conversion of type 'number' to type 'string' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 

tests/baselines/reference/asOperatorContextualType.errors.txt

@@ -1,10 +1,10 @@
-tests/cases/conformance/expressions/asOperator/asOperatorContextualType.ts(2,9): error TS2352: Type '(v: number) => number' cannot be converted to type '(x: number) => string'.
+tests/cases/conformance/expressions/asOperator/asOperatorContextualType.ts(2,9): error TS2352: Conversion of type '(v: number) => number' to type '(x: number) => string' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
   Type 'number' is not comparable to type 'string'.
 
 
 ==== tests/cases/conformance/expressions/asOperator/asOperatorContextualType.ts (1 errors) ====
     // should error
     var x = (v => v) as (x: number) => string;
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2352: Type '(v: number) => number' cannot be converted to type '(x: number) => string'.
+!!! error TS2352: Conversion of type '(v: number) => number' to type '(x: number) => string' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 !!! error TS2352:   Type 'number' is not comparable to type 'string'.

tests/baselines/reference/asOperatorNames.errors.txt

@@ -1,11 +1,11 @@
-tests/cases/conformance/expressions/asOperator/asOperatorNames.ts(2,9): error TS2352: Type 'number' cannot be converted to type 'string'.
+tests/cases/conformance/expressions/asOperator/asOperatorNames.ts(2,9): error TS2352: Conversion of type 'number' to type 'string' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 
 
 ==== tests/cases/conformance/expressions/asOperator/asOperatorNames.ts (1 errors) ====
     var a = 20;
     var b = a as string;
             ~~~~~~~~~~~
-!!! error TS2352: Type 'number' cannot be converted to type 'string'.
+!!! error TS2352: Conversion of type 'number' to type 'string' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
     var as = "hello";
     var as1 = as as string;
 

tests/baselines/reference/castingTuple.errors.txt

@@ -1,14 +1,14 @@
-tests/cases/conformance/types/tuple/castingTuple.ts(13,23): error TS2352: Type '[number, string]' cannot be converted to type '[number, string, boolean]'.
+tests/cases/conformance/types/tuple/castingTuple.ts(13,23): error TS2352: Conversion of type '[number, string]' to type '[number, string, boolean]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
   Property '2' is missing in type '[number, string]'.
-tests/cases/conformance/types/tuple/castingTuple.ts(14,15): error TS2352: Type '[number, string, boolean]' cannot be converted to type '[number, string]'.
+tests/cases/conformance/types/tuple/castingTuple.ts(14,15): error TS2352: Conversion of type '[number, string, boolean]' to type '[number, string]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
   Types of property 'length' are incompatible.
     Type '3' is not comparable to type '2'.
-tests/cases/conformance/types/tuple/castingTuple.ts(15,14): error TS2352: Type '[number, string]' cannot be converted to type '[number, string, boolean]'.
-tests/cases/conformance/types/tuple/castingTuple.ts(18,21): error TS2352: Type '[C, D]' cannot be converted to type '[C, D, A]'.
+tests/cases/conformance/types/tuple/castingTuple.ts(15,14): error TS2352: Conversion of type '[number, string]' to type '[number, string, boolean]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
+tests/cases/conformance/types/tuple/castingTuple.ts(18,21): error TS2352: Conversion of type '[C, D]' to type '[C, D, A]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
   Property '2' is missing in type '[C, D]'.
-tests/cases/conformance/types/tuple/castingTuple.ts(30,10): error TS2352: Type '[number, string]' cannot be converted to type '[number, number]'.
+tests/cases/conformance/types/tuple/castingTuple.ts(30,10): error TS2352: Conversion of type '[number, string]' to type '[number, number]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
   Type 'string' is not comparable to type 'number'.
-tests/cases/conformance/types/tuple/castingTuple.ts(31,10): error TS2352: Type '[C, D]' cannot be converted to type '[A, I]'.
+tests/cases/conformance/types/tuple/castingTuple.ts(31,10): error TS2352: Conversion of type '[C, D]' to type '[A, I]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
   Type 'C' is not comparable to type 'A'.
     Property 'a' is missing in type 'C'.
 tests/cases/conformance/types/tuple/castingTuple.ts(32,5): error TS2403: Subsequent variable declarations must have the same type.  Variable 'array1' must be of type '{}[]', but here has type 'number[]'.
@@ -30,21 +30,21 @@ tests/cases/conformance/types/tuple/castingTuple.ts(33,1): error TS2304: Cannot
     var emptyObjTuple = <[{}, {}]>numStrTuple;
     var numStrBoolTuple = <[number, string, boolean]>numStrTuple;
                           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2352: Type '[number, string]' cannot be converted to type '[number, string, boolean]'.
+!!! error TS2352: Conversion of type '[number, string]' to type '[number, string, boolean]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 !!! error TS2352:   Property '2' is missing in type '[number, string]'.
     var shorter = numStrBoolTuple as [number, string]
                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2352: Type '[number, string, boolean]' cannot be converted to type '[number, string]'.
+!!! error TS2352: Conversion of type '[number, string, boolean]' to type '[number, string]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 !!! error TS2352:   Types of property 'length' are incompatible.
 !!! error TS2352:     Type '3' is not comparable to type '2'.
     var longer = numStrTuple as [number, string, boolean]
                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2352: Type '[number, string]' cannot be converted to type '[number, string, boolean]'.
+!!! error TS2352: Conversion of type '[number, string]' to type '[number, string, boolean]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
     var classCDTuple: [C, D] = [new C(), new D()];
     var interfaceIITuple = <[I, I]>classCDTuple;
     var classCDATuple = <[C, D, A]>classCDTuple;
                         ~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2352: Type '[C, D]' cannot be converted to type '[C, D, A]'.
+!!! error TS2352: Conversion of type '[C, D]' to type '[C, D, A]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 !!! error TS2352:   Property '2' is missing in type '[C, D]'.
     var eleFromCDA1 = classCDATuple[2]; // A
     var eleFromCDA2 = classCDATuple[5]; // C | D | A
@@ -59,11 +59,11 @@ tests/cases/conformance/types/tuple/castingTuple.ts(33,1): error TS2304: Cannot
     // error
     var t3 = <[number, number]>numStrTuple;
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2352: Type '[number, string]' cannot be converted to type '[number, number]'.
+!!! error TS2352: Conversion of type '[number, string]' to type '[number, number]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 !!! error TS2352:   Type 'string' is not comparable to type 'number'.
     var t9 = <[A, I]>classCDTuple;
              ~~~~~~~~~~~~~~~~~~~~
-!!! error TS2352: Type '[C, D]' cannot be converted to type '[A, I]'.
+!!! error TS2352: Conversion of type '[C, D]' to type '[A, I]' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 !!! error TS2352:   Type 'C' is not comparable to type 'A'.
 !!! error TS2352:     Property 'a' is missing in type 'C'.
     var array1 = <number[]>numStrTuple;

tests/baselines/reference/contextualTyping39.errors.txt

@@ -1,9 +1,9 @@
-tests/cases/compiler/contextualTyping39.ts(1,11): error TS2352: Type '() => string' cannot be converted to type '() => number'.
+tests/cases/compiler/contextualTyping39.ts(1,11): error TS2352: Conversion of type '() => string' to type '() => number' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
   Type 'string' is not comparable to type 'number'.
 
 
 ==== tests/cases/compiler/contextualTyping39.ts (1 errors) ====
     var foo = <{ (): number; }> function() { return "err"; };
               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2352: Type '() => string' cannot be converted to type '() => number'.
+!!! error TS2352: Conversion of type '() => string' to type '() => number' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 !!! error TS2352:   Type 'string' is not comparable to type 'number'.

tests/baselines/reference/contextualTyping41.errors.txt

@@ -1,9 +1,9 @@
-tests/cases/compiler/contextualTyping41.ts(1,11): error TS2352: Type '() => string' cannot be converted to type '{ (): number; (i: number): number; }'.
+tests/cases/compiler/contextualTyping41.ts(1,11): error TS2352: Conversion of type '() => string' to type '{ (): number; (i: number): number; }' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
   Type 'string' is not comparable to type 'number'.
 
 
 ==== tests/cases/compiler/contextualTyping41.ts (1 errors) ====
     var foo = <{():number; (i:number):number; }> (function(){return "err";});
               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2352: Type '() => string' cannot be converted to type '{ (): number; (i: number): number; }'.
+!!! error TS2352: Conversion of type '() => string' to type '{ (): number; (i: number): number; }' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 !!! error TS2352:   Type 'string' is not comparable to type 'number'.

tests/baselines/reference/defaultArgsInFunctionExpressions.errors.txt

@@ -2,9 +2,9 @@ tests/cases/compiler/defaultArgsInFunctionExpressions.ts(4,19): error TS2345: Ar
 tests/cases/compiler/defaultArgsInFunctionExpressions.ts(5,1): error TS2322: Type 'number' is not assignable to type 'string'.
 tests/cases/compiler/defaultArgsInFunctionExpressions.ts(8,20): error TS2322: Type '3' is not assignable to type 'string'.
 tests/cases/compiler/defaultArgsInFunctionExpressions.ts(11,1): error TS2322: Type 'string' is not assignable to type 'number'.
-tests/cases/compiler/defaultArgsInFunctionExpressions.ts(14,51): error TS2352: Type 'string' cannot be converted to type 'number'.
+tests/cases/compiler/defaultArgsInFunctionExpressions.ts(14,51): error TS2352: Conversion of type 'string' to type 'number' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 tests/cases/compiler/defaultArgsInFunctionExpressions.ts(17,41): error TS2322: Type '""' is not assignable to type 'number'.
-tests/cases/compiler/defaultArgsInFunctionExpressions.ts(20,62): error TS2352: Type 'string' cannot be converted to type 'number'.
+tests/cases/compiler/defaultArgsInFunctionExpressions.ts(20,62): error TS2352: Conversion of type 'string' to type 'number' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 tests/cases/compiler/defaultArgsInFunctionExpressions.ts(28,15): error TS2708: Cannot use namespace 'T' as a value.
 
 
@@ -32,7 +32,7 @@ tests/cases/compiler/defaultArgsInFunctionExpressions.ts(28,15): error TS2708: C
     // Contextually type the default arg with the type annotation
     var f3 = function (a: (s: string) => any = (s) => <number>s) { };
                                                       ~~~~~~~~~
-!!! error TS2352: Type 'string' cannot be converted to type 'number'.
+!!! error TS2352: Conversion of type 'string' to type 'number' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 
     // Type check using the function's contextual type
     var f4: (a: number) => void = function (a = "") { };
@@ -42,7 +42,7 @@ tests/cases/compiler/defaultArgsInFunctionExpressions.ts(28,15): error TS2708: C
     // Contextually type the default arg using the function's contextual type
     var f5: (a: (s: string) => any) => void = function (a = s => <number>s) { };
                                                                  ~~~~~~~~~
-!!! error TS2352: Type 'string' cannot be converted to type 'number'.
+!!! error TS2352: Conversion of type 'string' to type 'number' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 
     // Instantiated module
     module T { }

tests/baselines/reference/fuzzy.errors.txt

@@ -2,7 +2,7 @@ tests/cases/compiler/fuzzy.ts(13,18): error TS2420: Class 'C' incorrectly implem
   Property 'alsoWorks' is missing in type 'C'.
 tests/cases/compiler/fuzzy.ts(21,34): error TS2322: Type 'this' is not assignable to type 'I'.
   Type 'C' is not assignable to type 'I'.
-tests/cases/compiler/fuzzy.ts(25,20): error TS2352: Type '{ oneI: this; }' cannot be converted to type 'R'.
+tests/cases/compiler/fuzzy.ts(25,20): error TS2352: Conversion of type '{ oneI: this; }' to type 'R' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
   Property 'anything' is missing in type '{ oneI: this; }'.
 
 
@@ -40,7 +40,7 @@ tests/cases/compiler/fuzzy.ts(25,20): error TS2352: Type '{ oneI: this; }' canno
             worksToo():R {
                 return <R>({ oneI: this });
                        ~~~~~~~~~~~~~~~~~~~
-!!! error TS2352: Type '{ oneI: this; }' cannot be converted to type 'R'.
+!!! error TS2352: Conversion of type '{ oneI: this; }' to type 'R' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
 !!! error TS2352:   Property 'anything' is missing in type '{ oneI: this; }'.
             }
         }