functions: Fix defaults mismatched types fallback

We allow primitive fallback values which have mismatched types, but only
if there is a conversion to the target type. Previously we would allow
unsafe conversions (e.g. string to bool), but later had no capacity to
return an error if the conversion failed due to the value of the
fallback being unable to convert to the target type.

This commit makes the more conservative requirement that default
fallback values must have a safe conversion.

lang/funcs/defaults.go

@@ -187,7 +187,7 @@ func defaultsAssertSuitableFallback(wantTy, fallbackTy cty.Type, fallbackPath ct
 		if fallbackTy.Equals(wantTy) {
 			return nil
 		}
-		conversion := convert.GetConversionUnsafe(fallbackTy, wantTy)
+		conversion := convert.GetConversion(fallbackTy, wantTy)
 		if conversion == nil {
 			msg := convert.MismatchMessage(fallbackTy, wantTy)
 			return fallbackPath.NewErrorf("invalid default value for %s: %s", wantTy.FriendlyName(), msg)

lang/funcs/defaults_test.go

@@ -389,6 +389,38 @@ func TestDefaults(t *testing.T) {
 				"c": cty.SetValEmpty(cty.String),
 			}),
 		},
+		// When specifying fallbacks, we allow mismatched primitive attribute
+		// types so long as a safe conversion is possible. This means that we
+		// can accept number or boolean values for string attributes.
+		{
+			Input: cty.ObjectVal(map[string]cty.Value{
+				"a": cty.NullVal(cty.String),
+				"b": cty.NullVal(cty.String),
+				"c": cty.NullVal(cty.String),
+			}),
+			Defaults: cty.ObjectVal(map[string]cty.Value{
+				"a": cty.NumberIntVal(5),
+				"b": cty.True,
+				"c": cty.StringVal("greetings"),
+			}),
+			Want: cty.ObjectVal(map[string]cty.Value{
+				"a": cty.StringVal("5"),
+				"b": cty.StringVal("true"),
+				"c": cty.StringVal("greetings"),
+			}),
+		},
+		// Fallbacks with mismatched primitive attribute types which do not
+		// have safe conversions must not pass the suitable fallback check,
+		// even if unsafe conversion would be possible.
+		{
+			Input: cty.ObjectVal(map[string]cty.Value{
+				"a": cty.NullVal(cty.Bool),
+			}),
+			Defaults: cty.ObjectVal(map[string]cty.Value{
+				"a": cty.StringVal("5"),
+			}),
+			WantErr: ".a: invalid default value for bool: bool required",
+		},
 	}
 
 	for _, test := range tests {