Merge pull request #8429 from Microsoft/declaredTypeAsInitialType

Declared type as initial type in control flow analysis

Author: ahejlsberg

src/compiler/checker.ts

@@ -7999,7 +7999,7 @@ namespace ts {
             const defaultsToDeclaredType = !strictNullChecks || type.flags & TypeFlags.Any || !declaration ||
                 getRootDeclaration(declaration).kind === SyntaxKind.Parameter || isInAmbientContext(declaration) ||
                 getContainingFunctionOrModule(declaration) !== getContainingFunctionOrModule(node);
-            const flowType = getFlowTypeOfReference(node, type, defaultsToDeclaredType ? type : undefinedType);
+            const flowType = getFlowTypeOfReference(node, type, defaultsToDeclaredType ? type : addNullableKind(type, TypeFlags.Undefined));
             if (strictNullChecks && !(type.flags & TypeFlags.Any) && !(getNullableKind(type) & TypeFlags.Undefined) && getNullableKind(flowType) & TypeFlags.Undefined) {
                 error(node, Diagnostics.Variable_0_is_used_before_being_assigned, symbolToString(symbol));
                 // Return the declared type to reduce follow-on errors

tests/cases/fourslash/quickInfoOnNarrowedType.ts

@@ -31,8 +31,8 @@ verify.quickInfoIs('(parameter) strOrNum: string');
 verify.completionListContains("strOrNum", "(parameter) strOrNum: string");
 
 goTo.marker('4');
-verify.quickInfoIs('let s: undefined');
-verify.completionListContains("s", "let s: undefined");
+verify.quickInfoIs('let s: string | undefined');
+verify.completionListContains("s", "let s: string | undefined");
 
 goTo.marker('5');
 verify.quickInfoIs('let s: string | undefined');